The present invention relates generally to ceramic to metal bonding and finds particular application in a ceramic arc discharge lamp.
Ceramic metal halide (CMH) lamps include a ceramic discharge vessel or “arc tube,” which is typically formed from polycrystalline alumina with small amounts of other additives. An arc discharge is generated by ionizing a fill material, such as a mixture of metal halide and mercury in an inert gas, such as argon, with an arc passing between two electrodes. In general, CMH lamps are operated on an AC voltage supply source with a frequency of 50 or 60 Hz, if operated on an electromagnetic ballast, or higher if operated on an electronic ballast. The discharge is extinguished, and subsequently re-ignited in the lamp, upon each polarity change in the supply voltage. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber, which maintains the pressure of the energized fill material and allows the emitted light to pass through. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being vaporized and excited by the electric arc. The electrodes are connected with a source of power by electrical conductors carried through tubular leg members of the discharge vessel. The conductors are typically formed from niobium, which has a similar coefficient of expansion to the ceramic used in forming the discharge vessel, and are hermetically sealed to the leg members with a seal glass, such as a dysprosia-alumina-silica glass.
The use of a seal glass to bond niobium to alumina places several design and processing constraints on the lamp. First, the seal glass has a maximum workable operating temperature of about 750° C. Additionally it is susceptible to corrosion by the rare earth elements in the fill. To minimize damage to the seals, they are positioned well away from the hottest part of the lamp, where the arc discharge forms. This governs the length of the legs, which must be long enough to sufficiently space the seals from the arc. This design results in a dead space in the legs of the discharge vessel which does not contribute to the light output yet which needs to be filled with the expensive halide dose. The length of the legs limits the ability for miniaturization and also renders the discharge vessel more prone to breakage in shipping. Additionally, the composition of the seal glass must be chosen carefully to match the thermal expansion characteristics of the conductors and ceramic, otherwise, the legs can crack during operation of the lamp. The seal glass position must be precisely controlled to minimize overlap with the molybdenum which is used to connect the tungsten electrode tips with the niobium conductors in order to avoid thermal expansion stresses. Finally, controlling arc gap requires crimping combined with careful time/temperature/pressure control in the drybox process to set desired electrode position.
The exemplary embodiment provides a discharge vessel and a method of forming a seal between alumina and metal which avoids the need to utilize a seal material.